Many plants respond to microbial attack by synthesizing and accumulating small lipophilic molecules called phytoalexins that have antibiotic activity1-3. The molecules that signal plants to begin this process of phytoalexin synthesis are called elicitors4T5. The hepta-P-D-g lucoside P-D-Glc-(1 --) 6)-[/3-~-Gl~-(l + 3)-Ip-D-Glc-(1 + 6)-/3-D-Glc-(1 + 6)-[P-D-Glc-(1 + 3)-]/i?-~-Glc-(l + 6)_~-Glc has been characterized as an elicitor of plant phytoalexin accumulation6. The compound was first purified to homogeneity from partial acid hydrolysates of isolated mycelial walls of the phytopathogen Phytophthora megasperma f. sp. glycinea6. Several structural analogues of the hepta-P-D-glucoside (listed in Fig. 1) have since been synthesized7,8 and tested for their biological activities'-". Early on it was shown that the presence of the o-glucose residue at the reducing end was not required for elicitor activity, as the chemical reduction of this residue, or even the complete absence of the group, did not affect the biological activity'. However, replacement of the side-chain p-0 + 3)-D-glucosyl residue of the terminal trisaccharide with an IV-acetyl+?-o-glucosaminyl (compound 6) or /3-o-glucosaminyl residue (compound 8) reduced the elicitor activity N 10 OOO-fold and -lo-fold, respectively'. The corresponding modifications of the nonreducing terminal backbone p-(1 --, 6)glucosyl residue (compounds 5 and 7) resulted in greater decreases in elicitor activity (_ 10 000-fold and N 100-fold)9, while substitution of this o-glucosyl residue with a D-xylosyl residue (compound 3) or o-galactosyl residue (compound 4) reduced the activity about lo-fold and 50-fold, respectivelylo~ll. Removal of the